2015年夏季北海道函館湾における有害渦鞭毛藻Karenia mikimotoiの初検出および2015–2016年の出現動態

抄録

This paper reports the first detection of Karenia mikimotoi cells and bloom occurrence in 2015 in Hakodate Bay, Hokkaido, Japan. We investigated the cell densities of K. mikimotoi in Hakodate Bay from May 2015 to November 2016. Regular samplings were conducted 1–3 times every month at the point at the wharf of Hakodate Research Center for Fisheries and Oceans (Stn. HKK). After the first detection of K. mikimotoi cells, water samples were collected every week. Karenia mikimotoi cells of 2 cells mL^-1 (0 m) and 1 cell mL^-1 (5 m) were first detected on 31 August 2015, and cell densities increased to form a red tide in November. Karenia mikimotoi cells were observed in the range of 96–630 cells mL^-1 (0 m), 5–272 cells mL^-1 (5 m) and 0–94 cells mL^-1 (9 m) from October to November 2015 at Stn. HKK. The water temperatures were in the range of 10.2–15.6℃ during this period. In 2016, the first detections of K. mikimotoi were 4 cells mL^-1 (0 m) and 12 cells mL^-1 (5 m) at Stn. HKK on 27 September, and the maximum cell density reached 34 cells mL^-1 at 5 m depth on 27 October. The cell densities of diatoms were low (<10³ cells mL^-1） in November 2015, because the amount of global solar radiation declined during November in the Hakodate area. The water temperature rapidly declined from late October to early November owing to the effects of a strong cold snap that hit Hokkaido in late October 2016. These adverse environmental conditions probably inhibited the growth of K. mikimotoi. Appearances of K. mikimotoi both in 2015 and 2016 suggested that K. mikimotoi cells were transported as a natural dispersal by the Tsushima/Tsugaru Warm Currents or via ships’ ballast water. There is a high probability that K. mikimotoi cells have appeared every year in Hakodate Bay until now and will appear hereafter.